The present invention relates to a two-stage anodic oxidation process for aluminum, which is employed as a support material for offset-printing plates.
Support materials for offset-printing plates are provided, on one or both sides, with a photosensitive coating (copying coating), either directly by the consumer, or by the manufacturers of precoated printing plates. This coating permits the production of a printing image by a photomechanical route. Following the production of the printing image, the coating-support carries the printable image-areas, and simultaneously there is formed, in the areas where there is no image (non-image areas), the hydrophilic image-background for the lithographic printing operation.
For the above reasons, the following requirements are demanded of a coating-support for photosensitive material for the manufacture of lithographic plates:
Those portions of the photosensitive coating which have become comparatively more soluble following exposure must be capable of being easily removed from the support, by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue. PA0 The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink. PA0 The photosensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure. PA0 The support material should possess good mechanical stability, for example, with respect to abrasion, and good chemical resistance, especially with respect to alkaline media. PA0 The direct current sulfuric acid process, in which anodic oxidation is carried out in an aqueous electrolyte which conventionally contains approximately 230 g of H.sub.2 SO.sub.4 per 1 liter of solution, for 10 to 60 minutes at 10.degree. to 22.degree. C., and at a current density of 0.5 to 2.5 A/dm.sup.2. In this process, the sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8 to 10% by weight of H.sub.2 SO.sub.4 (about 100 g of H.sub.2 SO.sub.4 per liter), or it can also be increased to 30% by weight (365 g of H.sub.2 SO.sub.4 per liter), or more. PA0 The "hard-anodizing process" is carried out using an aqueous electrolyte, containing H.sub.2 SO.sub.4 in a concentration of 166 g of H.sub.2 SO.sub.4 per liter (or about 230 g of H.sub.2 SO.sub.4 per liter), at an operating temperature of 0.degree. to 5.degree. C., and at a current density of 2 to 3 A/dm.sup.2, for 30 to 200 minutes, at a voltage which rises from approximately 25 to 30 V at the beginning of the treatment, to approximately 40 to 100 V toward the end of the treatment. PA0 The immersion treatment in aqueous solutions of TiF.sub.4, ZrF.sub.4, HfF.sub.4, or of corresponding complex acids or salts (see U.S. Pat. No. 3,440,050), PA0 The immersion treatment in aqueous solutions of silicates, bichromates, oxalates, or dyes (See U.S. Pat. Nos. 3,181,461 and 3,280,734), PA0 the immersion treatment in an aqueous solution of polyvinylphosphonic acid (See U.S. Pat. No. 4,153,461), PA0 the electrolytic treatment in an aqueous solution of sodium silicate (See U.S. Pat. No. 3,902,976), PA0 the partial detachment, in a first step, of the oxide layer, by means of aqueous acids or bases (e.g., by means of an aqueous solution of Na.sub.3 PO.sub.4) without the action of an electric current, or under cathodic electrolysis conditions, and the treatment, in a second step, with hot water or steam (See British Pat. No. 1,517,746), it being possible, in addition, for the water to contain dissolved salts in a quantity of up to 20% by weight (phospates or borates, among others), while its pH should lie within the range from 2 to 11, the treatment temperature being between 70.degree. and 130.degree. C., or PA0 a heat treatment, at 100.degree. to 300.degree. C., for approximately 1 minute, in dry air, or using steam (See German Offenlegungsschrift No. 2,716,604). PA0 European Patent Application No. 0,008,212 describes an electrolysis in a bath containing borate ions, prior to the anodic oxidation in a second bath (e.g. an aqueous H.sub.2 SO.sub.4 solution), the pH of the first bath to lie within the range from 9 to 11, and the treatment to be carried out at a temperature of 50.degree. to 80.degree. C.; it is desirable that the thickness of the first layer be at least 2 .mu.m, while that of the second layer should lie at higher values (e.g. about 20 .mu.m), PA0 British Pat. No. 1,523,030 describes an electrolysis in an aqueous solution of a salt (such as a borate or a phosphate) which contains, if appropriate, an acid or a salt as a barrier-layer forming agent (e.g., boric acid or ammonium borate). PA0 phosphoric acid (H.sub.3 PO.sub.4) PA0 sodium dihydrogen phosphate (NaH.sub.2 PO.sub.4) PA0 disodium hydrogen phosphate (Na.sub.2 HPO.sub.4) PA0 trisodium phosphate (Na.sub.3 PO.sub.4) PA0 phosphorous acid (H.sub.3 PO.sub.3) PA0 disodium phosphite (Na.sub.2 HPO.sub.3) PA0 diphosphoric acid (pyrophosphoric acid) (H.sub.4 P.sub.2 O.sub.7) PA0 sodium pyrophosphate (Na.sub.4 P.sub.2 O.sub.7) PA0 triphosphoric acid (H.sub.5 P.sub.3 O.sub.10) PA0 sodium triphosphate (Na.sub.5 P.sub.3 O.sub.10) PA0 polyphosphoric acid (H.sub.n+2 P.sub.n O.sub.3n+1) PA0 hexasodium tetrapolyphosphate (Na.sub.6 P.sub.4 O.sub.13) PA0 hexasodium metaphosphate (Na.sub.6 (PO.sub.3).sub.6) PA0 disodium monofluorophosphate (Na.sub.2 PO.sub.3 F) PA0 potassium hexafluorophosphate (KPF.sub.6). PA0 "Pure aluminum" (DIN Material No. 3.0255), i.e., composed of not less than 99.5% of Al, and the following permissible admixtures (maximum total 0.5%) of 0.3% of Si, 0.4% of Fe, 0.03% of Ti, 0.02% of Cu, 0.07% of Zn and 0.03% of other substances, or PA0 "Al-alloy 3003" (comparable with DIN Material No. 3.0515), i.e., composed of not less than 98.5% of Al, of the alloying constituents Mg, 0 to 0.3%, and Mn, 0.8 to 1.5%, and of the following permissible admixtures of 0.5% of Si, 0.5% of Fe, 0.2% of Ti, 0.2% of Zn, 0.1% of Cu and 0.15% of other substances. PA0 The layer-weight of the aluminum oxide, which is built up in the electrolyte containing H.sub.2 SO.sub.4, is either not adversely affected at all, or is affected only to a slight extent, as a result of which the mechanical strength (good resistance to abrasion) is preserved. PA0 The surface is lighter than in the case when the anodizing in the electrolyte containing H.sub.2 SO.sub.4 is the sole treatment, this increased lightness leading to improved contrast between image-areas and non-image areas on the printing-form. PA0 Qualitatively, the resistance to alkali is at least equivalent to that in an oxide layer which has been built up in an electrolyte containing H.sub.3 PO.sub.4 and, due to the larger layer thickness, is even quantitatively superior. PA0 The adsorption on the part of the oxide of, for example, dyes from the photosensitive coating is markedly reduced, or even suppressed, as a result of which it is possible to prevent the formation of "scumming" following the developing operation. PA0 The water-acceptance of the oxide, during printing, is improved in comparison to an oxide which has been produced only in stage (a); the number of copies which can be printed from one plate is comparable to the number which can be printed by conventional printing plates, i.e., by plates which have been anodically oxidized in a single-stage process, in electrolytes containing H.sub.2 SO.sub.4.
Aluminum is used, particularly frequently, as the base material for coating-supports of this type, the surface of this aluminum being roughened, according to known methods, by dry-brushing, wet brushing, sandblasting, or by chemical and/or electrochemical treatments. In order to increase the resistance to abrasion, substrates which have been roughened, especially by electrochemical treatments, are further subjected to an anodizing step, with the object of building up a thin oxide layer. These anodic oxidation processes are conventionally carried out in electrolytes such as H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, sulfamic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof. The oxide layers built up in these electrolytes or electrolyte mixtures differ from one another in structure, layer thickness and resistance to chemicals. In the commercial production of offset-printing plates, aqueous solutions of H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 are, in particular, employed.
By way of example, readers are referred to the following standard methods for the use of aqueous electrolytes, containing H.sub.2 SO.sub.4, for the anodic oxidation of aluminum (see, in this regard, e.g. M. Schenk, Werkstoff Aluminiun und seine anodische Oxydation [The Material Aluminum and its Anodic Oxidation], Francke Verlag, Bern, 1948, page 760; Praktische Galvanotechnik [Practical Electroplating], Eugen G. Leuze Verlag, Saulgau, 1970, pages 395 et seq., and pages 518/519; W. Huebner and C. T. Speiser, Die Praxis der anodischen Oxidation des Aluminiums [Practical Technology of the Anodic Oxidation of Aluminum], Aluminium Verlag, Duesseldorf, 1977, 3rd Edition, pages 137 et seq.):
Aluminum oxide layers, produced by these methods, are amorphous and, in the case of offset-printing plates, conventionally have a layer-weight of approximately 1 to 8 g/m.sup.2 corresponding to a layer thickness of approximately 0.3 to 2.5 .mu.m. The oxide layers are distinguished by a fine channel-like structure; they possess good mechanical stability as a result of which they protect, in particular, the structure of electrochemically roughened aluminum against abrasion. The oxide layers produced in H.sub.2 SO.sub.4 electrolytes possess a comparatively low resistance to alkaline solutions, which are used to an increasing extent, for example, in the processing of presensitized offset-printing plates, and which are used preferentially in up-to-date developing solutions for exposed photosensitive coatings working either negatively or, in particular, positively. This comparatively low resistance to alkaline solutions is a disadvantage when a carrier material which has been anodically oxidized in this way is used for offset-printing plates.
Also known is the anodic oxidation of aluminum in aqueous electrolytes containing oxygen acids of phosphorus, or containing phosphates.
A process for manufacturing a lithographic printing plate is described in U.S. Pat. No. 3,511,661, in which process the aluminum support is anodically oxidized in a 42, 50, 68 or 85% strength aqueous H.sub.3 PO.sub.4 solution, at a temperature of at least 17.degree. C., until the layer of aluminum oxide has a thickness of at least 50 nm.
A process is known from U.S. Pat. No. 3,594,289, in which a printing-plate support material, composed of aluminum, is anodically oxidized in a 50% strength aqueous H.sub.3 PO.sub.4 solution, at a current density of 0.5 to 2.0 A/dm.sup.2 and a temperature of 15.degree. to 40.degree. C.
The process for the anodic oxidation of aluminum supports, in particular for printing plates, according to U.S. Pat. No. 3,836,437 is carried out in a 5 to 50% strength aqueous Na.sub.3 PO.sub.4 solution, at a current density of 0.8 to 3.0 A/dm.sup.2, a temperature of 20.degree. to 40.degree. C., and for a duration of 3 to 10 minutes. The aluminum oxide layer thus produced, is stated to possess a weight of 10 to 200 mg/m.sup.2.
According to U.S. Pat. No. 3,960,676, the aqueous bath for the electrolytic treatment of aluminum which is thereafter to be provided with a water-soluble or water-dispersible coating substance, contains 5 to 45% of silicates, 1 to 2.5% of permanganates, or borates, phosphates, chromates, molybdates or vanadates, in concentrations ranging from 1% up to saturation.
The anodic oxidation of printing-plate support-materials, composed of aluminum, is also described in British Pat. No. 1,495,861. This oxidation is carried out in a 1 to 20% strength aqueous solution of H.sub.3 PO.sub.4, or of polyphosphoric acid, employing alternating current at a current density of 1 to 5 A/dm.sup.2 at 10.degree. to 40.degree. C.
Another support material for printing plates is known from British Pat. No. 1,587,260. This material carries an oxide layer which is produced by the anodic oxidation of aluminum in an aqueous solution of H.sub.3 PO.sub.3, or in a mixture of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.3, after which a second oxide film, of the "barrier-layer" type, is additionally super-imposed on this relatively porous oxide layer. It is possible for this second oxide layer to be formed by anodic oxidation in aqueous solutions containing, for example, boric acid, tartaric acid, or borates. Both the first stage (Example 3, 5 minutes) and the second stage (Example 3, 2 minutes) are carried out very slowly, the second stage being carried out, moreover, at a comparatively high temperature (80.degree. C.).
Admittedly, an oxide layer produced in these electrolytes is frequently more stable with respect to alkaline media than an oxide layer which has been produced in an electrolyte based on a H.sub.2 SO.sub.4 solution. It additionally exhibits a number of other advantages, such as a lighter surface, better water-acceptance or low adsorption of dyes ("scumming" in the non-image areas), but it nevertheless possesses significant disadvantages. In a modern belt-type unit for the manufacture of printing-plate supports, it is possible, employing voltages and residence-times which are technically appropriate, to produce oxide-layer weights ranging, for example, up to only approximately 1.5 g/m.sup.2, a layer thickness which naturally offers less protection against mechanical abrasion than a thicker layer of the type produced in a H.sub.2 SO.sub.4 electrolyte. Due to the fact that the pore volume and the pore diameters are larger in an oxide layer which has been built up in H.sub.3 PO.sub.4, the mechanical stability of the oxide itself is also lower, which results in further losses with regard to abrasion-resistance.
Processes have also been disclosed which attempt to combine the advantages of the two electrolytes, in that electrolyte mixtures composed of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4 are employed, or a two-stage treatment procedure takes place.
The process for manufacturing printing-plate support-materials, composed of aluminum, in accordance with British Pat. No. 1,410,768 is carried out in a manner wherein the aluminum is initially anodically oxidized in an electrolyte containing H.sub.2 SO.sub.4, and this oxide layer is then subjected to a follow-up treatment in a 5 to 50% strength by volume aqueous H.sub.3 PO.sub.4 solution, without the action of an electric current. The actual oxide layer is stated to possess a superficial weight of 1 to 6 g/m.sup.2 ; however, this weight decreases significantly on immersion in the aqueous H.sub.3 PO.sub.4 solution, for example, by approximately 2 to 3 g/m.sup.2 per minute of immersion-time in an aqueous H.sub.3 PO.sub.4 solution. It is stated that an electrochemical treatment in the H.sub.3 PO.sub.4 solution is also possible (Example 11), or that it should be possible to employ a mixed electrolyte, composed of H.sub.3 PO.sub.4 and H.sub.2 SO.sub.4 (Example 12). A removal of the oxide layer is said to also occur in these cases.
Similar processes, in which, however, the treatment with the aqueous H.sub.3 PO.sub.4 solution is effected exclusively without the influence of an electric current, can also be found in U.S. Pat. No. 3,808,000, or in British Pat. No. 1,441,476. In addition, in German Offenlegungsschrift No. 2,548,177, or in U.S. Pat. No. 3,940,321, there is described a two-stage electrochemical treatment, initially in an electrolyte based on H.sub.2 SO.sub.4, and then in an electrolyte based on H.sub.3 PO.sub.4.
U.S. Pat. Nos. 4,049,504 and 4,229,266 describe a mixed electrolyte, composed of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4, for the manufacture of printing-plate support-materials. The latter patent additionally mentions a specific content of aluminum ions.
In European Patent Applications Nos. 0,007,233 and 0,007,234, support materials for aluminum printing plates are anodically oxidized in a process whereby they initially run, as middle conductors, through a bath containing aqueous H.sub.3 PO.sub.4 and an anode, and then run into a bath containing aqueous H.sub.2 SO.sub.4 and a cathode. The two electrodes can also be connected to a source of alternating voltage. It is also indicated, but not specified further, that the treatment with H.sub.3 PO.sub.4 could be a simple immersion treatment, or that it would even be possible to substitute neutral or alkaline solutions for the acids.
Although the processes with mixed electrolytes, with increasing H.sub.3 PO.sub.4 content, cause the properties of the oxide to be approximated to the properties obtained by an anodic oxidation in pure aqueous H.sub.3 PO.sub.4 solutions, they nevertheless never reach these properties. On the other hand, the positive properties of an anodic oxidation in pure aqueous H.sub.2 SO.sub.4 solutions (oxide-layer thickness, abrasion-resistance) also decline. Moreover, a bath-monitoring procedure (in the case of a solution with several components) is very expensive in terms of production technology, and is difficult to control. The two-stage anodic oxidation, or treatment method, leads to a situation wherein the oxide layer which has been built up in the H.sub.2 SO.sub.4 electrolyte is redissolved in the H.sub.3 PO.sub.4 solution to an excessive extent under the conditions hitherto known.
The following after-treatment steps for aluminum which has been anodically oxidized in an aqueous H.sub.2 SO.sub.4 solution are also known in the field of printing-plate support materials:
Of these after-treatment methods, only the silicatizing and the boehmite formation (reaction with H.sub.2 O at an elevated temperature) lead to a certain improvement in the resistance of the oxide layers to alkalis. In the case of silicatizing, however, a deterioration can occur in the storage life of presensitized (ready-coated) printing plates, and the treatment to form boehmite can be carried out only with increased difficulty in modern, fast-running belt-type units, since it requires a comparatively long treatment time (exceeding 1 minute, e.g., 5 minutes). Moreover, boehmite formation can lead to a deterioration in the adhesion of the layer.
Occasional publications also describe methods whereby certain surface-modifications are even carried out before the anodic oxidation in H.sub.2 SO.sub.4 solutions, for example:
However, both publications refer only to aluminum which is to be employed for window frames, plates (panelling components) and fastening devices for architectural structures, or to decorative aluminum moldings for vehicles or household articles. Moreover, the formation of thinner layers would lead to the possibility of their being redissolved too easily during the second treatment.
In British Pat. No. 1,412,929, an aluminum surface is treated with hot water or steam (with the formation of a layer of boehmite), and an electrolysis is thereafter carried out, as a further treatment, in an aqueous solution of a salt of silicic acid, phosphoric acid, molybdic acid, vanadic acid, permanganic acid, stannic acid, or tungstic acid. This treatment is intended to lead to a greater layer thickness, improved toughness, a finer structure, and hence to greater corrosion-resistance (e.g. against acids or alkali). A similar process is also described in U.S. Pat. No. 3,945,899, where the surface of the aluminum may be in the form not only of a layer of boehmite, but may also be a chemical "conversion layer" resulting from a treatment employing a chromate or a phosphate. In the examples, the durations of the electrolysis treatment lie within the range from 2 to 10 minutes. However, both treatment-steps are too protracted for modern belt-type units and, moreover, the aluminum coatings, produced by non-electrolytic methods, are less suited to the practical requirements which are demanded of high-performance printing plates (e.g., with regard to abrasion-resistance and interactions with the photosensitive coating).